RAF after BoB: mid-term strategy, tactics technology?

While several aircraft did change from V-12s to radials the Merlin to Hercules swap is going to be a bit harder.



As Mr. Brooks says, there is a lot more Merlin below the prop shaft than above. Not so with the Hercules.



Ki-61 had a rather low prop


Ki-100



Now maybe you could get it to work. Longer landing gear and move the guns out-board in the wing, requiring a new wing guns bay area. Hump back the fuselage so pilot can see over the engine, this may or may not interfere with the airflow over the vertical stabilizer and rudder requiring bigger Stab/rudder? Needs new reduction gear on the engine to run smaller/faster turning propeller. And so on and so on...........

And after all that work what have you got?

A plane that is heavier than the one you started with and has more drag (even 8-12%)?

And since you sucked up thousands (if not tens of thousands) of man hours engineering this conversion what don't you get? two stage Merlin Spitfires? Griffion Spitfires?

Edgar Brooks said:

The Spitfire fuselage is 36" wide; the Hercules is 55" diameter, so it won't fit. If you butcher the airframe, to get it in, and keep the same thrust-line, the top of the engine would be so high, the pilot couldn't see over it, or use a gunsight. Lower the thrust-line, so he can see, and you can't have a propeller bigger than 8' diameter.
The Spitfire III was cancelled because the Merlin XX was needed for the four-cannon ground-attack Hurricane, which was needed to replace the obsolescent Whirlwind; the Spitfire III also needed a completely new set of cowlings (like the Hurricane II it was 4" longer,) while the Merlin 45, with only minor adjustments to the carburettor intake, could fit into the same space as the Merlin II/III.
The broader-chord rudder was first fitted to the XII, not the IX, but extended onto other Marks later.

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Where is it at its widest? Over the cam covers?

wuzak said:

Where is it at its widest? Over the cam covers?

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Frame 5 (engine bulkhead/firewall.)

If putting a radial in a Spitfire was not practical, what about putting a radial in a P-40?

Uh, I think they called that a P-36

Or Hawk 75.

And we have several threads on that one

Something to note, of the planes that did swap between radials and in-line engines ALL had cowl guns firing over the top of of the engine which means they might have had more fuselage depth (top to bottom) than the Spitfire. Or at least more distance from the pilots eyes to the prop shaft.

Hello,

Edgar Brooks said:

The Spitfire fuselage is 36" wide; the Hercules is 55" diameter, so it won't fit.

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The engine is always 'connected' to the airframe via an engine bearer. Bearer is always tailored to fit to the firewall/bulkhead on the one side, and obviously the engine on the another. Wide engines were made to fit slender airframes historically, as we can see, for example, on the Ki-61 and Ki-100 drawings Shortround6 kindly posted.

If you butcher the airframe, to get it in, and keep the same thrust-line, the top of the engine would be so high, the pilot couldn't see over it, or use a gunsight. Lower the thrust-line, so he can see, and you can't have a propeller bigger than 8' diameter.

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There will be no butchering
The Germans installed a distinctively low-thrust DB-605 on a captured Spit V, the prop was of 3 m diameter (9.84 ft); all worked fine.

The Spitfire III was cancelled because the Merlin XX was needed for the four-cannon ground-attack Hurricane, which was needed to replace the obsolescent Whirlwind; the Spitfire III also needed a completely new set of cowlings (like the Hurricane II it was 4" longer,) while the Merlin 45, with only minor adjustments to the carburettor intake, could fit into the same space as the Merlin II/III.

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The space where the smaller Merlin 45 fitted, later accommodated the longer, two-speed two-stage Merlins.
The Merlin XX was installed in the Hurricane (12 gun version) to cut a bit the performance advantage that Bf-109E have had during the BoB?

The broader-chord rudder was first fitted to the XII, not the IX, but extended onto other Marks later.

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Thanks.

edit: Flight magazine states that diameter of the Hercules was 52 inch.

tomo pauk said:

There will be no butchering

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Hercules in a Spitfire is the very definition of butchery!

tomo pauk said:

The Germans installed a distinctively low-thrust DB-605 on a captured Spit V, the prop was of 3 m diameter (9.84 ft); all worked fine.

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The DB 605 (or was it a DB 601) Was about the same dimensions as a Merlin. Teh thrust line was lower, but at a guess I woud say no lower than the Griffon.

A Hercules will have a lower thrust line and require a smaller prop.

tomo pauk said:

The space where the smaller Merlin 45 fitted, later accommodated the longer, two-speed two-stage Merlins.
The Merlin XX was installed in the Hurricane (12 gun version) to cut a bit the performance advantage that Bf-109E have had during the BoB?

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The Merlin XX was the same width and height as the 45 - just longer.

The 2 stage Merlins were wider by about 0.7" but lower by 3". But Much longer.

The Griffon was about 0.5" wider than the Merlin 45 and about 3" taller. And also longer.

The Griffon in the XII weighed slightly less than the Hercules, while the Griffon 65 weight slightly more (not including cooling systems). The Griffon II/VI weighed about 100lbs more than the Merlin 66, yet the XII was 500lbs heavier than the LF.IX. The Griffon 65 weighed 250-300lb more than the Merlin 66, but the Spitfire XIV was 1500lb heavier than the LF.IX. Some of that would be to do with the extra cooling, but also with the required strengthening of the airframe.

tomo pauk said:

Hello,The engine is always 'connected' to the airframe via an engine bearer. Bearer is always tailored to fit to the firewall/bulkhead on the one side, and obviously the engine on the another. Wide engines were made to fit slender airframes historically, as we can see, for example, on the Ki-61 and Ki-100 drawings Shortround6 kindly posted.

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The Ki-100 conversion was made possible because Kawasaki could study the Fw 190 installation, a luxury the British did not have in 1941. As it was, the conversion was forced on Kawasaki because of the almost total failure of the DB derived Ha-140, meaning there were lots of engineless Ki-61 airframes sitting around at a time when Japan desperately needed all the fighters it could get. AFAIK, the British didn't have a desperate need to alter the Spitfire to take a radial engine.

tomo pauk said:

There will be no butchering
The Germans installed a distinctively low-thrust DB-605 on a captured Spit V, the prop was of 3 m diameter (9.84 ft); all worked fine.

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Of course there will be butchering and a lot of alterations. First of all the aerodynamics around the forward fuselage would be completely butchered, unless the designers came up with a radically different cowling for the Hercules, and a proper set of fairings for the forward fuselage. Redesigning the cowling and developing proper fairings alone would require hundreds of man-hours, and a complete change of British design philosophy. Sure, Sydney Camm, for example, came up with some excellent designs for the Centaurus in the Tempest II and Fury series, but that was three years later, after examining the likes of the Fw 190.

The main engine bulkhead would have to be redesigned to cater for the completely different engine mounts of the Hercules.

Any thoughts on what's going to happen to the airflow over the inner wing and horizontal tail when the cowl flaps are opened, noting also the lower thrust line of the Hercules?

Any thoughts as to how the inefficient collector exhaust system - almost universal on British radials in 1941 - would penalise the Spitules' performance? Would the exhaust fumes be properly dispersed away from the cockpit area?

tomo pauk said:

The space where the smaller Merlin 45 fitted, later accommodated the longer, two-speed two-stage Merlins.The Merlin XX was installed in the Hurricane (12 gun version) to cut a bit the performance advantage that Bf-109E have had during the BoB?

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As noted by Wuzak, the Merlin 60/70 series was only marginally wider than the 45 - the fit was a whole lot more comfortable than trying to fit a radial engine, plus its cowling.

The question again: why fit the Hercules? There would have been absolutely no advantage, and a whole lot of trouble for...what?

Anything that keeps the Hercules from being fitted on the front of the Spitfire can be fixed with enough time and effort.

With the same amount of time and effort you can mount several other engine options. You only have so many engineers and draftsmen.

Why have them work on what is going to be the 3rd or 4th best option?

Options.

1. Fit 2 speed Merlin XX, Mr. Brooks says it needed 4 more inchs, some accounts claim about 50 Spitfires were modified later in the war to take Merlin XX series engines and used for air/sea rescue duties. These were well used Spitfires that had their original single speed engines taken out.
Given that you can pry Merlin XX engines away from Bomber Command or from the Hurricane II this is your easiest conversion. Least amount of new parts and flight testing needed.
2. Fit the two Stage Merlin as was done.
3. Fit the Griffon. As was done. If you have spare engineers/draftsmen have them work on this. The Mark IV DP845 first flew on 27 November 1941. However it needed some development to sort out.
"Jeffrey Quill, Supermarine's chief test pilot, was the first to fly the Mk IV/Mk XII prototype DP845; "...there was somewhat less ground clearance, resulting in a slight reduction in propeller diameter; the power available for take-off was much greater; and the engine RPM were lower than in the Merlin. All this meant that the throttle needed to be handled judiciously on take-off but, once in the air, the aeroplane had a great feeling of power about it; it seemed to be the airborne equivalent of a very powerful sports car and was great fun to fly. Changes of trim with changes of power were much more in evidence, both directionally and longitudinally, and the aeroplane sheared about a bit during tight manoeuvres and simulated dog-fights. I realised at once that we should have to correct its directional characteristics and probably its longitudinal stability also, both of which in due time we achieved. Indeed, DP485 eventually went through many phases of development throughout and I, and others, flew in it a great deal; it became one of our favourite aeroplanes."

4. The Hercules is NOT going to be a snap fit.
You have about the same RPM as the Griffon except the normal props are for bombers and are way too big. Yes you can fit smaller props and perhaps change the gear ratio= more work.
You need to sort out the intake to the carb, can't use existing ones= More work.
You need to sort out the center of gravity. without having the radiators as counter weights =more work.
Your view over the nose is going to be truly horrible. Even more work won't solve this one unless you move the entire cockpit up and humpback the fuselage like the Italian fighters. Spitfire was known to have a worse veiw over the nose than the Hurricane.
To have any hope of a low drag cowling you have to toss aside everything Bristol has done and start over. Without a low drag cowling the idea doesn't work. How much work and experimentation is needed? And while a good NACA cowling may be better than Bristol's cowling in 1941 even a 1941 NACA cowling is a far cry from what good cowling would be several years later.
Maybe you can get Bristol to flip the supercharger intake and fit an up-draft carburetor. It is certainly not impassable but it is one more thing that needs work.
Bristol did eventually design a rearward flow exhaust system with exhaust thrust. SO it could be done, the question is when and at what cost (in terms of other work Bristol was doing)

The list just goes on.

What do you get in the end?

Shortround6 said:

What do you get in the end?

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A great thread.

tomo pauk said:

The engine is always 'connected' to the airframe via an engine bearer. Bearer is always tailored to fit to the firewall/bulkhead on the one side, and obviously the engine on the another. Wide engines were made to fit slender airframes historically, as we can see, for example, on the Ki-61 and Ki-100 drawings Shortround6 kindly posted..

Click to expand...

Frame 5 (the bulkhead/firewall) was behind the wings' leading edges, and more-or-less level with the mainspars (to which it was bolted.) You either have to cut away several inches of wing (where it's most efficient,) or extend the engine bearers enough to keep the engine clear of the wing. This plays havoc with balance/CoG (the IX needed counter-balancing lead weights in the rear fuselage, and the XIV had them in the fin.

The space where the smaller Merlin 45 fitted, later accommodated the longer, two-speed two-stage Merlins.

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In width and height only; the IX fuselage was 9" longer than that of the I/II/V. In fact the 45 was not smaller than the III or XX, it was actually slightly longer, but a tweak to the carburettor position made it the same overall length, so it could still fit.

The Merlin XX was installed in the Hurricane (12 gun version) to cut a bit the performance advantage that Bf-109E have had during the BoB?

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The Hurricane was in danger of being totally outclassed (in 1942 Malta refused to accept any more, even the Mk.II,) yes, so it got priority for the Merlin XX.

edit: Flight magazine states that diameter of the Hercules was 52 inch

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With, or without, cowling? (And it still won't fit a 36" fuselage.)

You need a couple of feet behind the main diameter of the cylinder heads for the supercharger, carb, and accessories (generators, pumps,etc).
This 'stuff' fills the space between the Bulkhead and leading edge.
However I agree that the needed work far exceeds the work needed to fit a different V-12. And for a questionable result.
Higher drag means more fuel burn for SAME speed. Shorter ranges for SAME fuel.

wuzak said:

...The DB 605 (or was it a DB 601) Was about the same dimensions as a Merlin. Teh thrust line was lower, but at a guess I woud say no lower than the Griffon.

A Hercules will have a lower thrust line and require a smaller prop.
...

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The thrust line for the DB Spitfire was lower than the main fuselage longeron (pic taken from airwar.ru; can be translated; scroll down for photos 3-view of greater resolution) :

Edgar Brooks said:

...

[about Hercules being at 52 in diameter:]With, or without, cowling? (And it still won't fit a 36" fuselage.)

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Not specified, more likely it is without cowling.

The Propeller used on that aircraft is the same diameter as used on the Bf 109. Maybe they could have used a slightly larger one.

The point remains, though, that the thrust line would be lower with the Hercules, requiring a similar or smaller prop.

Note that the Fw 190A's prop was 3.3m (10 ft. 11¾ in) in diameter - a full foot larger in diameter than in the DbSpit. Also note that the the BMW 801 was more tightly cowled and slightly smaller in diameter.

Luftwaffe Resource Center - Fighters/Destroyers - A Warbirds Resource Group Site

The Beaufighter TF.X with Hercules XVII (developed from the VI) had propellers of 12ft 9in diameter - nearly 2 feet bigger than on the Fw 190.

Bristol Beaufighter

A smaller prop would require more blades and a faster rotation speed. A note on increasing the prop rpm (by increasing engine rpm limits) on the Beaufighter:

In level flight in M.S. supercharger gear, there is a small increase in speed (2 1/2 m.p.h.) due to the increased r.p.m. and boost limitations. In F.S. gear, however, there is a decrease in speed of about 1 1/2 m.p.h., above full throttle height at the new limitations. It wouild appear that the increase of power due to increasing engine r.p.m. is small and that the decrease in propeller efficiency at the higher r.p.m. produces a nett decrease in thrust horse power in this case. It seems doubtful in view of these results, whether the increased R.P.M. limitations are worth while, in view of the probable decrease in engine life.

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Beaufighter (F) Mk.VI Testing

I still think that if you are going to stick a radial in any fighter it has to be the Hurricane. That frees up Merlin XXs for the Spitfire - either as the III or an interim MkV/MXX version.

As the Hurricane was being pushed into 2nd line roles already by 1941, the radial may help with its survivability.

It would also help Camm with the development of the radial cowling for the Tempest II in later years.

And the Hurricane already had a larger prop than the Spitfire, so it may be less compromised in that area.

Double post

I cannot understand this obsession with radial engines; the Hurricane is the same as the Spitfire, with a fuselage too narrow, and the firewall behind the wings' leading edges and level with the wingspars, so you're just not going to get the Hercules in.
There is an added complication, since the Hurricane was more touchy, with regard to CoG balance (they couldn't fit pilot's armour to the early Mk.I with two-blade prop, and had to have the three-blade fitted first.)
The Hurricane was not relegated to secondary duties (unless you feel that the defence of Malta, the desert, and Atlantic convoys was secondary,) in fact the whole idea of fitting the Merlin XX + 20mm cannon was because of the expectation of the Battle of Britain restarting with the better weather; we know, now, that it didn't happen, but that wasn't known in December 1940. There's also the consideration that it was expected that any fighting would be between 20,000'-30,000' (the 109s were flying above 20,000' before the BoB ended,) and the Hercules ran out of "puff" long before that.
Fitting the Hercules (or any other radial) into the Hurricane would not have freed the Merlin XX for the Spitfire, since the decision to fit the Merlin 45 (virtually a "drop-in" fit, which the XX was not) had been taken as early as 15-12-1940.

They managed to bolt radial engines onto a couple of Bf 109s, and what a waste of time that was.

Both the Spitfire and Hurricane were designed for what became known as the Merlin engine (the PV-12 first ran in 1933) and I just don't understand why anyone would want to bolt a radial onto them. Evidently the British didn't see the point at the time either

Cheers

Steve

It's good to think outside the box, or the cowling, so to speak.
May not always achieve a usable idea, but then again, once in a while, something marvelous may result.

Well, more than 57,000 Hercules were produced by the end of the war and thankfully not a single one was bolted onto a Spitfire! Something marvellous I don't think it would have been

Cheers

Steve

I dont get it either. The RAF had problems 41-2, as it tried to take the fight to Germany, but that wasnt due to the Merlin. They were still playing catch up in the experience of their pilots, they had poor tactical options over france, and technology wise, their fighters lacked range, but I dont see how a hercules is going to change that