F.6/42-type fighter, but with Merlin power (1 Viewer)

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BarnOwlLover

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Nov 3, 2022
Mansfield, Ohio, USA
F.6/42 was a competition for a "lightweight fighter" that was intended to be Britain's answer to the Fw190 concept. This was ultimately won by the Hawker Fury and the Folland Fo.117, though only the Sea Fury would see worthwhile production for the Royal Navy.

F.6/42 recommended the use of the Rolls-Royce Griffon, Napier Sabre or Bristol Centaurus engines. But what could a F.6/42 type fighter be like if the Merlin was encouraged as a possible power plant type? Remember that F.6/42 was written when the two-stage Merlin was brand new, and was seen as a 1600-1700 hp engine and was thought to be entering its final stretch of development. 2000+hp versions of the Merlin weren't seen as being likely at the time until the 100 series was developed, which came later.

The considerations are as follows:

Tier 1: Maneuverability, speed, climb rate, firepower, pilot and systems protection.

Tier 2: Ability to serve as a ground attack aircraft with 1000-1500 lbs of bombs, possibility to scale this up to 2000 lbs in the future if more powerful Merlins can be fitted. Also he ability to carry 6-8 HVARs or up to 8 (16 if paired) RP-3 rockets.

Tier 3: Range. Long range for possibiliy of bomber escort or standing patrols or recon is desired, but not a priority.

In addition to Merlin power, the aircraft is hoped to have an armament of 6 (or maybe up to 8) .50 MGs with 300 (min) or 400 (ideal) rounds per gun, or 4 20mm cannons with 150 (min) or 200 (ideal) rounds per gun.
 
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A lot depends on what the predicted power of the Merlin was in late 1942/early 1943 when work was done on the initial proposals.
Predicted power is always going to be higher than the current service power, but it may be lower than the actual power obtained by production versions years down the road.
Work was being done on 150 octane fuel in 1943.
The next problem is at what altitude do you want the power.
A Merlin using 150 octane fuel and 25lbs of boost is very powerful down low but the superchargers cannot supply the needed air above 12,000ft for a Merlin 66 engine. Power falls from 12,000ft until at 17,000ft power has dropped to the power at 18lbs. And that is in high speed level flight. The drop in power is even more severe in climb due to the lack of RAM.
Perhaps as Merlin 70 will offer better results but the Merlin 70 won't offer quite the same power because the supercharger take more power to drive with the higher gear ratio.

A lot depends on the altitude you want to fight at. The later Merlins didn't offer much in the way of power over a Merlin 66 or 70 running on 150 octane fuel. What they did offer was increased durability/reliability due to stronger engine cases, improved crankshaft and oiling system.
A 100 series Merlin is not going to offer 25lbs at much over 23,000ft. The supercharger won't do it.
The engine in the P-51H was good for 1930hp at 10,100ft using 25lbs of boost in low gear and 1630hp at 23,500ft at 25lbs of boost in high gear.

If you can get what you want with around 1600hp in the low 20s from the Merlin then it may work, if you need more power you need the bigger engines.
 
Looking like the 'baby Fury' perhaps? British know by this time how to make good, streamlined radiators in front of the main spar. Oil cooler can go under nose, together with oil tank. Use heat from radiators to heat the guns, these being the 4 Hispanos. Fuel system - 100 gals in front of the pilot, two drop tanks outboards of the U/C attachment points, L-shaped fuel tank under and behind of the pilot of 60 gals.
Yes, the less fuel is poured in, the better the RoC and maneuverability.
The slanted decking from windscreen to the engine bay for better visibility.
Fowler flaps might be nice. Blown canopy, steeper widscreen than on Spiftire. Fully covered U/C when retracted. Might want to split the ram air intakes so these can be tucked in the wings.

Another option is to mimic the Mustang, it checks all the boxes. British take on the topic will probably be a bit lighter, too?
 
All of this sounds good, but I do wonder why the Merlin would be called a dead end when even the most advanced versions of the Mustangs used them, the DH Hornet used them (though the two-stage Griffon was being worked on by then), and even the Spiteful was being looked at as being Merlin powered in case of issues with the two-stage Griffon.

Maybe the Merlin RM.17SM might be a point to look at, though in 1942/43 probably wasn't yet a thought.
 
All of this sounds good, but I do wonder why the Merlin would be called a dead end when even the most advanced versions of the Mustangs used them, the DH Hornet used them (though the two-stage Griffon was being worked on by then),
Who proclaimed Merlin as dead-end?
 
OK, show me the Merlin powered fighter that was going to carry

Tier 1: Maneuverability, speed, climb rate, firepower, pilot and systems protection.
the aircraft is hoped to have an armament of 6 (or maybe up to 8) .50 MGs with 300 (min) or 400 (ideal) rounds per gun, or 4 20mm cannons with 150 (min) or 200 (ideal) rounds per gun.
Also he ability to carry 6-8 HVARs or up to 8 (16 if paired) RP-3 rockets.
Tier 3: Range. Long range for possibiliy of bomber escort or standing patrols or recon is desired, but not a priority.

Pick what you don't want and use the Merlin, or pick most of the list and find the bigger engine.

Pointing out that the Merlin until you get the last ones, didn't change power that much once you got into the high teens or low 20s. The increases in power from the better fuel only worked at low altitude.
Also please note that the 30 minute climb rating often did not change and the take-off power did not change. Only the 5 minute rating (or a bit longer in combat).
R.M.14.S.M Was rated at 12lbs at 2850rpm for 30 minutes
R.M.16.S.M Was rated at 12lbs at 2850rpm for 30 minutes
The R.M.16.S.M, due to it's higher supercharger gears, was limited to much less power for take-off than the R.M.14.S.M. at the same pressure.

Use the P-51H as a guide. No 20mm cannon. just six 50s and around 300rpg. Certainly not 16 rockets.

The P-51s were good dog fighters, not great. You might not get the flexibility to swap between roles ( I can swap the rockets for more fuel for escort) as easily at it seems.

What is the desired radius when carrying a pair of 1000lb bombs? Just saying it could carry big bombs without saying how far gives a lot of leeway.

Typhoons did carry big bombs, it also took quite a bit of work to get it to go well. Work that was going on in 1943. (new tires and brakes and a new prop and bigger tail planes)
 
Mustang X, but based on the Mustang Ia (= 4 cannons) with wings rated for drop tanks or underhung ordnance might fit the bill.
Spitfire is always an option, Mk.VIII with 4 cannons as a starting point.
G.55/G.56/Re.2005, but with Merlin 60 series in the nose instead of the DB, with extra pair of cannons.
Big-wing Fw 190, again with Merlin 60 in the nose, 4 cannons outboard of the wheels' bay.
P-40L, but the same engine in the nose, 4 cannons instead of 6 HMgs.

P-40 might have the intercooler radiator inside the wings, like the -Q2 had the coolant radiators. Italian machines can have the intercooler radiator istallation like the Mosquito that was powered by 2-stage Merlins.
 
The Mustang had no problems carrying large bomb or rocket loads when used as a ground attack aircraft. Only issue is that used in the ETO it wasn't an insanely good interceptor (even then it could outclimb most Me-109 variants and most Fw-190 variants) and wasn't as agile as the Spitfire (which was lighter, in part due to different build standards, and also carried a lot less fuel), which that didn't matter much provided that it was a better dogfighter than most 109s and 190s by the time it entered service, also helped by Luftwaffe pilot skills and ability deteriorating rapidly as the Mustang pilots inflicted attrition from Big Week onwards.

Of course, this could've been remedied by the lightweight Mustangs entering production, the P-51B/C/D/K getting more powerful engines (as the P-51B used by Rolls-Royce as an engine test bed) and maybe improved control surfaces used on the lightweight Mustangs, or the P-51H (built to British design standards, improved controls, and more power).

Or a fresh fighter based on the concept of "half a DH Hornet" that weighed half as much (Hornet F1 weighed 14,180 lbs clean, Hornet F3/Sea Hornet F20 a bit more; figures from the books Hornet & Sea Hornet: de Havilland's Ultimate Piston Engine Fighter and Airframe Album #8: The de Havilland Hornet and Sea Hornet), which IMO shows that a fresh design incorporating all of that could be possible, and that even an improved Mustang based on the LW designs or even the P-51H would at least come close to fulfilling that.

Though it should be noted that I did list design priorities different tiers. Top tier being most important, bottom tier being least important.
 
OK, show me the Merlin powered fighter that...
I'm not sure I can agree with any of this.

The altitude argument is a strange one because 1: The Merlin absolutely can and did perform well at high altitudes and 2: The actual aircraft that resulted from F.6/42 (the Sea Fury) wasn't actually a particularly good high-altitude performer. Granted part of that may be because it only ever entered service as a naval fighter and thus was probably geared for lower altitudes. Furies with different versions of the Centaurus or the Sabre might, theoretically, have been different, but this seems unlikely when you consider the land-based Tempests from which the Fury derives (and which were, in practice, actually quite similar in performance). The Tempest V reached critical altitude in the high teens (the same as the Sabre VII in the prototype Fury I), and fell off precipitously thereafter, just as you suggest is true of the Merlin. The Tempest II reached critical altitude lower (though higher than Sea Fury FB.11), albeit with a more gradual drop off above that. In comparison to your chosen Merlin example, the P-51H (which had a critical altitude above 20k), the Tempest derivatives were 15-25 mph slower at 20k and 25-35mph slower at 30k (the Fury FB.11 being the fastest at 20k but the slowest at 30k). A Fury with a Centaurus geared more like the Tempest II (which has a slightly higher critical altitude in FS), might have been a bit closer, but not by much and certainly no better. The Fury I with a Sabre VII would have been comparable, if they could get it to work reliably, but sleeve valve engines tend to have problems above about ~15 lbs boost, so I'm sceptical they'd have managed to get it into service — at least not before it was overtaken by Merlin development, which was by that point a much safer bet (someone mentioned RM.17SM, which by the time the Fury I was cancelled was producing 2200 hp reliably, and as much as 2600 hp with ADI, comparable to the Centaurus). All in all it seems apparent that a properly developed (and geared) Merlin, in a well designed air frame is in fact at least as well suited to high altitude combat as the design that actually resulted from F.6/42, to whatever degree that was actually relevant to the specification (which I'm not sure it was).

Of course I need hardly point out that the P-51H offers decidedly superior range to any of the above, and I must question why you'd think that would be any different given comparable external stores. Unfortunately I can't find numbers for the Sea Fury's combat radius with external stores exactly comparable to those of P-51H, but with a pair of 110 US Gallon drop tanks (roughly comparable in size and weight to a 500 lb bomb) it has a combat radius of 1002 miles, compared to 720 miles for an FB.11 with a pair of 90 Imperial gallon (108 US gallon) tanks. I realise this is an imprecise mode of comparison, but as the volumes of additional fuel carried, are similar, and the size and weight comparable to a common bomb load, I think it's useful as a very general indicator. Note that both have the same max bomb load of two 1000 lb bombs. While the Fury is able to carry 12 HVARS to the Mustang's 10, both are in excess of the specification's requirements.

Retrofitting the Mustang for a cannon armament wouldn't have been difficult. As others have pointed out, many of the original RAF Mustangs were delivered with cannon and while the P-51H wing was different in both planform and I believe structure (if memory serves this is where they found much of the weight savings in the H), I can't imagine this would have been a significant obstacle. Performance would likely have taken a bit of a hit thanks to the extra drag, as even the shorter Mk V Hispanos probably wouldn't fit flush, but I doubt it would have made enough of a difference to make the P-51H uncompetitive vs the Sea Fury given the margin between them.

Earlier versions of the Mustang were indeed not dog fighters, but the P-51H, being considerably lighter and considerably more powerful would have been much more effective, probably at least comparable to the Sea Fury and quite possibly superior if wing loading is anything to go by — though really either aircraft's speed would mean pilots would rarely be forced into dogfights they didn't want to be in anyway.

To be honest, I think a cannon armed, Merlin 100 engined "Mustang VI" would have been considerably superior to the Sea Fury in most respects, and probably pretty comparable to a hypothetical production Fury I, depending on each aircraft's degree of development. The advantage of the Mustang is that it would have been much cheaper, and available sooner. The advantage of the (Sea) Fury is that it keeps Hawker's production lines humming, which has strategic and political value.
 
The Mustang had no problems carrying large bomb or rocket loads when used as a ground attack aircraft. Only issue is that used in the ETO it wasn't an insanely good interceptor (even then it could outclimb most Me-109 variants and most Fw-190 variants) and wasn't as agile as the Spitfire (which was lighter, in part due to different build standards, and also carried a lot less fuel), which that didn't matter much provided that it was a better dogfighter than most 109s and 190s by the time it entered service, also helped by Luftwaffe pilot skills and ability deteriorating rapidly as the Mustang pilots inflicted attrition from Big Week onwards.

Of course, this could've been remedied by the lightweight Mustangs entering production, the P-51B/C/D/K getting more powerful engines (as the P-51B used by Rolls-Royce as an engine test bed) and maybe improved control surfaces used on the lightweight Mustangs, or the P-51H (built to British design standards, improved controls, and more power).

Or a fresh fighter based on the concept of "half a DH Hornet" that weighed half as much (Hornet F1 weighed 14,180 lbs clean, Hornet F3/Sea Hornet F20 a bit more; figures from the books Hornet & Sea Hornet: de Havilland's Ultimate Piston Engine Fighter and Airframe Album #8: The de Havilland Hornet and Sea Hornet), which IMO shows that a fresh design incorporating all of that could be possible, and that even an improved Mustang based on the LW designs or even the P-51H would at least come close to fulfilling that.

Though it should be noted that I did list design priorities different tiers. Top tier being most important, bottom tier being least important.
I've always been intrigued by the idea of a "half Hornet," not least because I imagine it would be pretty gorgeous. Considering the Hornet itself was already in the early stages of development (privately) by the time F.6/42 was issued, and the necessary production equipment was already basically there for the Mosquito, I suspect they could have gotten a damn good fighter into service pretty quickly, albeit likely at the expense of at least some Mosquito production, which may not have been worth it given that designs utility and quality. A "half-mosquito" (i.e. no "slimline" Merlin, and reusing the RAF 34 profile wing rather than developing a new laminar flow airfoil) perhaps makes more sense in production terms, at the cost of a bit more drag, but that still leaves you with a plane about as light as a Spitfire, but with a drag profile closer to a Mustang, and you can make in vast numbers without using too much aluminum.

Also, as concerns the Spitfire's weight, I don't think you should discount telescoping wing spar trickery. A major pain in the ass from a manufacturing perspective, but for an aircraft with such a large, thin (which counterintuitively, usually means heavy) wing to have been so light is no small accomplishment.
 
The Fury I with a Sabre VII would have been comparable, if they could get it to work reliably, but sleeve valve engines tend to have problems above about ~15 lbs boost, so I'm sceptical they'd have managed to get it into service — at least not before it was overtaken by Merlin development, which was by that point a much safer bet (someone mentioned RM.17SM, which by the time the Fury I was cancelled was producing 2200 hp reliably, and as much as 2600 hp with ADI, comparable to the Centaurus). All in all it seems apparent that a properly developed (and geared) Merlin, in a well designed air frame is in fact at least as well suited to high altitude combat as the design that actually resulted from F.6/42, to whatever degree that was actually relevant to the specification (which I'm not sure it was).

The RM.17SM produced 2,600hp only on the test bench, and not cleared for flight at that power.

The RM.17SM was rated at 2,200hp in MS gear and 2,100hp in FS gear. I can't recall the altitudes for these ratings, but the FS gear was ~15,000ft FTH. It was, in Merlin terms, a "low" altitude engine.


To be honest, I think a cannon armed, Merlin 100 engined "Mustang VI" would have been considerably superior to the Sea Fury in most respects, and probably pretty comparable to a hypothetical production Fury I, depending on each aircraft's degree of development. The advantage of the Mustang is that it would have been much cheaper, and available sooner. The advantage of the (Sea) Fury is that it keeps Hawker's production lines humming, which has strategic and political value.

So, the P-51H?
 
I'm not sure I can agree with any of this.
Go back and look at the original posters "requirements".

The Sea Fury carried 145 rounds per gun of 20mm ammo and that was 363lbs. Now 150 rpg is not off but much but 200rpg? an extra 137lbs.
The Sea Fury carried 200imp gallons of fuel.

The Tempest II carried 636 rounds. The Tempest II held 160 imp gal. The Tempest II in Jan 1945 was using an engine that gave 1950hp at 16,500ft. using 8lbs of boost, that is max airflow. The engine will do 12lbs of boost with 150 octane but at lower altitude. Please note the engine was rated at 2300hp at 5,000ft at 8lbs in low gear. It was taking several hundred HP to drive the supercharger in high gear more than in low hear. The intake charge was also being heated more and you actually get less air at the same pressure. That is the problem with just re-gearing the supercharger for higher altitude. It can be done and was done but there was always a cost.

The Merlin was great engine but I tried to point out that comparing the "sprint" rating (5 minute) rating does NOT mean that the other ratings also follow. Raise a Merlin from 18lbs boost to 25lbs boost and the 30 minute rating does not change, neither do the max rich cruise and the max lean boost. If you want a plane that can handle heavy loads in a lot of different flight conditions you need an engine with more power for 30 minute climbs, max rich cruise and max lean cruise.
 
Just looking at power isn't telling the full story though. Yes the Centaurus makes more power at cruise than the Merlin, but the Sea Fury was also correspondingly larger and heavier, so the specific excess thrust available to each aircraft for things like carrying bombs may not have been so different after all as it initially seems. In fact, as specific excess thrust tends to vary pretty directly with climb performance, we can be confident that at least for takeoff, the Mustang actually had the advantage on the Sea Fury — hence it's being able to carry the same 2000 lb bomb loads. Now at each aircraft's 30 minute max, or under cruising settings perhaps that advantage would dissipate, or even shift in the Sea Fury's favour. It also might not; I don't have numbers to hand for each aircrafts power under those settings, or much of a sense how much drag a pair of 1000 lb bombs are adding, so I can't do the math to say for sure. But the Mustang can clearly take off with the same bomb load and fly a considerable (likely greater) distance with it; even if the Sea Fury can climb a bit faster or turn a bit better with that bomb load under lower engine settings, I'm not sure how useful that really is in practice; both aircraft have sufficient performance to take off with the same bomb load and carry it a good distance, and if either aircraft gets bounced, the pilot is hardly gonna keep the engine at max rich cruise, he's gonna up the boost and then there's the Mustang's performance advantage back again (assuming it ever left).
 
I did some digging and found that a Sea Fury X has a max continuous cruise power of 1450hp in full supercharger at ~18k, compared with 1190hp at ~24k for a Merlin 130 (aside from ancillary positioning, probably more or less identical to what you'd want in a hypothetical F.6/42 Mustang). Adding a ~2200 lb load (GP bombs usually being a little heavier than their nominal weights, plus the weight of the racks) to each aircraft's loaded weight , that gives you a power/mass of .104 for the Mustang, and .101 for the Sea Fury; practically identical with maybe a slight edge to the Mustang. Now of course this doesn't account for prop efficiency (which I imagine would be comparable) or drag (which I imagine would favour the mustang), both of which are significant factors that can't simply be discounted. However it gives us at least a rough idea of how both are performing under cruise settings with what appears to be their maximum bomb load. Keep in mind also that the Merlin 130 is geared for higher altitudes than the Centaurus XVIII; if both were geared for the same altitude, the Mustang would probably have more of an advantage.

What this suggests to me is that the Sea Fury, while an undoubtedly an excellent aircraft, was perhaps bigger and more powerful than a front-line tactical fighter at that time actually needed to be. A smaller, lighter, more streamlined aircraft with a more powerful engine relative to its size and weight can clearly offer comparable ordinance loads, comparable or better flight performance, better fuel economy, and will likely do it all at a lower cost (and in the case of the sleeve-valve Bristol, with lower maintenance requirements). Is the lightweight Mustang the best approach to that design brief? Not necessarily (the idea of a "half-Hornet" admittedly intrigues me, and I'm sure one could come up with other solutions as well), but it's probably about as close as you'll get in terms of actually existing designs.
 
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One of the issues of the "half-Hornet" that I've wondered about is layout of things like the radiators, landing gear, and other items given that it'll have one engine and (typically) one pilot (I'd like like for there to be a dual control trainer that could also maybe be used for other roles). We do have to remember that the Hornet packed in almost 1500 miles range on internal fuel (Hornet 1 at econ speed), 4x20mm cannons with 190 rpg, and ability to carry 2x1000 lb bombs or 8 RP-3 rockets into a plane that had a clean take off weight as low as just above 14,000 lbs.

Now, if you take the "half-Hornet" approach, you can have a plane that weights a bit over 7000 lbs, can be just as fast, with a 700+ mile range on internal fuel, still carry 4x20mm cannons (maybe with somewhat reduced or perhaps similar ammo loadout), and carry 8 RP-3s or a 1000 lb bomb load. But this is also hypothetical given that such an aircraft didn't really exist.
 
Now, if you take the "half-Hornet" approach, you can have a plane that weights a bit over 7000 lbs, can be just as fast, with a 700+ mile range on internal fuel,
What is the math behind the 700 mile range for e the 'half Hornet' on internal fuel?
 
I think it could certainly be done, especially if you went with a laminar flow wing like the actual Hornet had (though this would take more development vs reusing the Mosquito profile). Granted, I don't really know the Hornet's airfoil that well, but if it's anything like the Mustang's profile, the wing will reach it's thickest point further back on the chord, which in theory means more room to recess cannon deeper in the wing, and more space forward of the main spar (which you'd assume would be at the thickest part of the wing) to house a radiator with adequate room behind it for hot air to expand.

Other alternatives could be a something like a "mini Tempest," scaling down the Tempest to the Merlin with either wing or annular radiators as Hawker had been experimenting with, or possibly retrofitting the Spiteful wing/radiator configuration to the Merlin Spitfire frame (a Mk 21 wing with Mk III style radiators could be a more conservative approach). Any of these could potentially benefit as well from repositioning of the engine ancillaries as in the 130 series for a closer cowling and fewer intakes hanging off the side (though this would likely have to be slightly different on low-wing fighters). In general, the concept relies on the absolute minimisation of weight and drag, since it's the power (be it at take off, cruise, or combat engine settings) left over after countering those forces that largely determines the utility of the aircraft — everything from ordinance load, to range, to max rate of climb. The benefit of pursuing a lightweight Merlin (or indeed Griffon) powered tactical fighter over heavier alternatives like the Tempest/Fury is that because of the boost limitations of the sleeve valve, designs based on the Sabre and the Centaurus were running into the hard upper limits of specific excess thrust, whereas the Merlin still had room to grow, as it simply ate boost for breakfast. Big American radials had similar development potential to the Merlin, and it's not surprising that they were the focus of US development, but at that point they were not without their own (ultimately surmountable, but still significant) teething problems, and may not have actually offered particularly better specific excess thrust once fully developed.

The problem is that the main two Merlin designs in production weren't (yet) up to par with the latest single-engined heavies in this regard — the Spitfire because it was too draggy, and the Mustang because it was a fat fuck. For take off and combat, where maximum power is the ticket, this could be mitigated to some degree by simply feeding the Merlin more breakfast (though cutting weight obviously helps in both regards), but at cruise it required reductions in drag, as a V-1650-9 or even an RM.17SM probably aren't going to produce massively more power at their most economical settings than a 60 series Merlin. The P-51H offered that low drag, as well as an appreciable reduction in weight, which is why I think it's a pretty decent option, but it's still a good 2000 lbs heavier than a Merlin Spit or a hypothetical half-Hornet, so there's room to imagine a still lighter aircraft which would have been better in combat and able to lift more ordinance (provided weight savings didn't unduly compromise structurally integrity of course).
 
What is the math behind the 700 mile range for e the 'half Hornet' on internal fuel?
I would imagine simply by cutting the Hornet F.1's range in half. You could arrive at an (at least marginally) more thorough, and possibly more conservative number by comparing the internal fuel of a similarly sized aircraft (say a later Merlin Spit or perhaps a Yak-9) with the fuel consumption of a Hornet.
 
I would imagine simply by cutting the Hornet F.1's range in half. You could arrive at an (at least marginally) more thorough, and possibly more conservative number by comparing the internal fuel of a similarly sized aircraft (say a later Merlin Spit or perhaps a Yak-9) with the fuel consumption of a Hornet.

Thank you.
I'll wait for the answer, though :)
 

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